Various semiconductor laser structures have been devised and realized. An important characteristic for a semiconductor laser is its threshold current. When currents exceeding that threshold pass through a laser, laser oscillation and emission of coherent radiation can be expected. High threshold current, however, tends to reduce the laser lifetime. In order to reduce the threshold current to a minimum value, many semiconductor laser structures employ a current concentration means. The current concentration means generally constricts current flow to a localized area within the laser, reducing the total current flow needed to reach the threshold current density in the localized area. Inclusion of current concentration means can increase the complexity of the structure of the laser, its manufacturing process, and its cost.
A cross-sectional, perspective, and schematic view of a known laser structure including current confinement means is shown in FIG. 3. That v-channeled substrate inner stripe (VSIS) laser is described in Volume 40 of Applied Physics Letters, pages 372-374 (1982). In the conventional laser of FIG. 3, a p-type GaAs substrate 1 has disposed on it an n-type GaAs current blocking layer 2. A p-type lower cladding layer 3 of Al.sub.y Ga.sub.1-y As is disposed on current blocking layer 2. A p-type Al.sub.x Ga.sub.1-x As active layer 4 is disposed on lower cladding layer 3. An n-type Al.sub.y Ga.sub.1-y As upper cladding layer 5 is disposed on active layer 4. In a typical laser structure according to FIG. 3, x is approximately 0.15 and y is approximately 0.49. An n-type GaAs contacting layer 6 is disposed on upper cladding layer 5. Electrodes 11 and 12 are disposed on the exposed surfaces of substrate 1 and contacting layer 6, respectively.
The laser structure of FIG. 3 includes a v-shaped longitudinal groove 13 that extends along the length of substrate 1, generally parallel to the interfaces between semiconductor layers, and confines current flowing through the laser (i.e., from one electrode to the other) to the central stripe region defined by the groove 13. Groove 13 extends generally normal to the interfaces between semiconductor layers, through current blocking layer 2 and into substrate 1. Groove 13 is filled with a portion of lower cladding layer 3 so that cladding layer 3 is the only substantially non-planar layer in the structure.
The VSIS laser of FIG. 3 can be manufactured with only two crystalline growth steps. In the initial growth step, current blocking layer 2 is grown on substrate 1. That structure is removed from the crystalline growing apparatus and groove 13 is formed in blocking layer 2 and substrate 1 by conventional photolithography and chemical etching techniques. After the formation of groove 13, the second crystalline growth step is carried out. AlGaAs layers 3, 4, and 5 and GaAs contacting layer 6 are successively grown, preferably by liquid phase epitaxy (LPE). Deposition of electrodes 11 and 12 complete manufacture of the laser structure. Thus, the manufacturing process is relatively simple.
When the laser of FIG. 3 is properly biased so that an electric current flows between electrodes 11 and 12, much of the potential current flow is blocked by the reverse biased pn junction formed between current blocking layer 2 and first cladding layer 3. However, current can easily flow through groove 13 since at the area of its penetration of substrate 1 no rectifying junction is present. Typically, groove 13 is three to five microns wide where it penetrates substrate 1.
As indicated by the arrows in FIG. 3, the current flowing from substrate 1 through groove 13 spreads laterally in the groove and into layer 3. The component of the current flowing toward electrode 12 largely contributes to laser oscillation and the production of light. However, that portion of the current that flows laterally in layer 3 from groove 13, i.e., the reactive current, does not contribute to laser oscillation. Likewise, the lateral flow of charge carriers in active layer 4 opposite groove 13 does not contribute to laser oscillation and increases the reactive current component of the current flow. Because of the portion of the total current that is lost to reactive current flow, the threshold current of a laser of the type shown in FIG. 3 is relatively high, e.g., 35 to 45 milliamps.
Another conventional laser structure employing current confinement means is shown in a perspective, schematic, cross-sectional view in FIG. 4. In the buried v-channel substrate inner stripe (B-VSIS) laser structure of FIG. 4, the current confinement means of FIG. 3 is supplemented to suppress reactive current flow. As in all figures, like elements of FIGS. 3 and 4 are given the same reference numerals. In addition to the elements previously described for FIG. 3, the structure of FIG. 4 includes an n-type GaAs cap layer 7 disposed opposite groove 13 and a relatively high resistance Al.sub.z Ga.sub.1-z As buried layer 8 disposed on current blocking layer 2. A second buried layer 9 of p-type Al.sub.u Ga.sub.1-u As is disposed on layer 8 adjacent contacting layer 6. In a typical structure, z is approximately 0.8 and u is approximately 0.2. High resistance layer 8 is disposed at opposite sides of groove 13 to suppress the lateral flow of charge carriers from groove 13.
The production process for the laser structure of FIG. 4 is more complicated than that of FIG. 3 and requires three crystalline growth steps. In an initial crystalline growth step, current blocking layer 2 is deposited on the substrate 1. Thereafter, as described for the structure of FIG. 3, groove 13 is formed by conventional photolithography and etching steps. In the second crystalline growth step, lower cladding layer 3, active layer 4, upper cladding layer 5, and cap layer 7 are successively deposited, preferably by LPE. In order to shape the resulting structure for the growth of layers 8 and 9, conventional photolithography and etching techniques are employed to remove portions of layers 3, 4, 5, and 7. That removal results in the formation of a longitudinal mesa as would be indicated in FIG. 4 if layers 8, 9, and 6 were absent. In the third crystalline growth step, high resistance AlGaAs layer 8, p-type AlGaAs layer 9, and contact layer 6 are successively grown. As before, electrodes 11 and 12 are then deposited by conventional techniques to complete the structure.
When a current flows through the laser structure of FIG. 4, lateral current flow in layers 3, 4, 5, and 7 is suppressed by layers 8 and 9. Reactive current flow in the active layer 4 that does not contribute to laser oscillation is thereby reduced and the threshold current of the laser is reduced to about 20 to 25 milliamps.
While the structure of FIG. 4 achieves a significant reduction in threshold current, the complexity of the manufacturing techniques required significantly increases costs. Three crystalline growing steps are required as compared to two in the structure of FIG. 3. The mesa formation between the second and third crystalline growth steps requires high precision in photolithography and etching. Moreover, the regrowth interfaces between the longitudinal sides of the mesa, particularly layer 4, and layers 8 and 9 that are formed at the sides of the mesa, i.e., the laser oscillation region, are inherently weak and result in reduced laser lifetime.